This invention relates to container closures. It has particular application to crown caps for bottles, most especially to beer bottle crowns.
A beer container or package must protect the flavor of the beer during transport and storage of the beer. It must exclude all oxygen. It must not contain any materials which can be extracted by the beer, and conversely it must not scavenge the flavorants naturally occurring in the beer. Although bottled beer has popularly been regarded as well protected, brewers have long known that bottle crowns have not provided altogether adequate protection of the beer. The problem has been found to be largely in the plastic crown liner. A discussion of the problems associated with polymeric packaging materials (including crown liners) in the beer industry is contained in a paper entitled "Packaging Materials and Beer Quality" by Dr. W. A. Hardwick, Jr., appearing as chapter 23 in Beer Packaging: A Manual for the Brewing and Beverage Industries, edited by Harold M. Broderick (Madison, Wisconsin, 1982).
The most common crown liner material is polyvinyl chloride (PVC), containing dioctylphthalate as a plasticizer, and a calcium or zinc stearate antioxidant. This material scavenges some of the flavor-giving esters in beer and frequently contains extractable impurities.
The PVC liner is applied either as a creamy plastisol and spread by spinning or molding, or else by melting a powdered plastisol in an extruder and extruding the molten material into the crown shell. In the latter process, a tamping tool may be used to form the liner. The liner is sometimes applied as a die-cut film and melted to the crown shell in situ. In any of these processes, it is important to control the temperature carefully to drive off all volatile materials without damaging the crown shell or the polymer. The crown liner is sometimes foamed to give it more resilience, but both the foaming agent and the increased surface area of the liner increase the likelihood that the liner will affect the flavor of the beer.
Other crown liners have been used or proposed, but all have similar problems. For example, polyethylene and ethylene vinyl acetate liners have an even greater effect than PVC, by scavenging flavor components from the beer. In U.S. Pat. No. 3,799,380 to Hashimoto et al, a bottle crown liner is formed by reacting, in the crown shell, a polyol and an aromatic isocyanate to form a thermoset polyurethane. This approach requires new equipment to replace the standard PVC liner and requires that the bottle crown manufacturer maintain the highest manufacturing standards to assure complete reaction of the monomers and complete removal of solvents and catalysts. Even when the process is carried out with utmost care, the process generally requires the use of an excess of toxic isocyanates, which are reacted after the main polymerization reaction has been completed. Moreover, a substantial curing period is required, preferably at elevated temperature. The resulting thermoset polyurethane liner may have physical and chemical properties which are not altogether desirable. An earlier patent to Mahoney, No. 3,442,411, discloses a similar approach, with a foamed polyurethane.